1. Field of the Invention
The invention relates to seismic source arrays.
2. Description of the Related Art
Seismic data are usually acquired using arrays of seismic sources. In a source array, individual seismic sources are arranged in a certain spatial pattern. The most common marine seismic sources are airguns but also vibrators, waterguns and steam-injection guns are in use. The most common land seismic sources are vibrators and dynamite charges. Seismic source arrays are usually made up of one type of source. The sizes and strengths of the individual sources within the array may be different. In addition, the individual sources can be made to fire or start emitting at the same time or with small time delays between them.
In marine seismic surveys, the source array is usually towed by a vessel. A typical configuration is shown in FIG. 1, in which a vessel 2 tows an airgun source array 4. In land seismic surveys, a vibrator is mounted on a truck; a dynamite charge is placed in a drilled hole.
An individual source has three spatial positions: in-line, cross-line and depth. In the marine example in FIG. 1, the cross-line separation of the airguns is 8 m, the in-line separation is 3 m and their depth is 6 m.
The design of a source array amounts to the selection of the number of individual sources, their strengths, their signatures, their positions (in-line, cross-line and depth) and their firing/emission delays. The design criteria are based on the desired strength and frequency content at the geological) target depth and a desire to radiate energy principally downward.
The source arrays that are commonly used, exhibit source array directivity. This means that they do not emit the same seismic signal in all directions. The emitted signal can vary with azimuth (angle) and take-off angle. The concepts of azimuth and take-off angle are explained in FIG. 2, in which the vessel 2 and source array 4 are again shown. The present specification is only concerned with azimuth.
FIG. 3 shows the directivity pattern of the source array in FIG. 1. At frequencies 90 Hz and 130 Hz the directivity of the source array is clearly varying with azimuth. The directivity in azimuth decreases for lower frequencies as is shown at frequencies 60 Hz and 20 Hz. FIGS. 4a, b and c show the seismic signal and its amplitude and phase spectrum emitted at a take-off angle of 30° and at a range of azimuths. The change in the signal shape, its amplitude spectrum and its phase spectrum is significant.
The presence of azimuthal directivity in the seismic data is undesirable. During seismic data processing seismic data traces from different azimuths are combined to give the final image. Azimuthal directivity will have a detrimental effect: it results in a loss of resolution and a reduction of the signal-to-noise ratio.
A distinction can be made between two types of marine seismic acquisition:    (a) Sea-surface acquisition, in which a vessel tows one or more cables with built-in receivers. The receiver cables are usually towed at a depth between 3 m and 12 m. This is the most common type of acquisition and is usually referred to as towed-streamer acquisition.    (b) Sea-floor acquisition, in which the receivers are planted at the sea floor or built into a receiver cable, which is laid at the sea floor. This type of acquisition is a relatively recent development.
In both types of acquisition the sources are usually located at or near the sea-surface. The source array in FIG. 1 would be typical for both sea-surface and sea-floor acquisition.
In both types of acquisition the source vessel, which might be the same vessel that is towing the receiver cable in sea-surface acquisition, sails through the survey area and activates the source at regular intervals. In 2D acquisition a single cable (called a streamer) is towed behind the vessel, while in 3D acquisition, an array of parallel streamers, normally equally spaced apart, is towed behind the vessel.
In 3D sea-floor acquisition, the receiver cables 6 (see FIG. 5) are laid out in an area over which the source vessel 2 sails a 3D pattern. Thus, seismic data are recorded in all directions from the source 4 (see FIG. 5), that is for a full circle of azimuths: 0°-360°.
In 3D sea-surface acquisition the receiver cable 8 is usually towed behind the source vessel 2; a technique called end-on acquisition (see FIG. 6). Thus, during one sail line, the seismic data are recorded for a half-circle of azimuths −90° to +90°. In fact, because the streamer is longer (typically, 4 km to 8 km) than the cross-line offset of the outer streamers (typically, 200 m to (500 m), much of the data have an azimuth fairly close to 0°. Occasionally, receiver cables are towed both in-front-of and behind the source vessel; a technique called split-spread acquisition. Then, the seismic data are recorded for an entire circle of azimuths although much of the data have an azimuth close to either 0°, for the receiver cable behind the source vessel, or 180°, for the receiver cable ahead of the source vessel.
The source arrays that are used in sea-floor acquisition are the same as the ones used in sea-surface acquisition. These were originally designed for 2D towed-streamer acquisition in which data are only acquired straight behind the vessel at a single azimuth of 180°. The directivity in azimuth was therefore of no concern. As discussed, 3D sea-surface seismic data contain a fan of azimuths and 3D sea-floor seismic data contain all azimuths. The azimuthal directivity of the source array will therefore be present in the data.
In land seismic acquisition, source arrays are usually formed by placing a number of land seismic vibrators in a spatial pattern. The acquisition geometry of a 3D land survey is similar to the sea-floor acquisition geometry as shown in FIG. 5, but with the receiver cables at the earth's surface. Thus, 3D land seismic data are acquired for all azimuths and the azimuthal directivity of the source array is present in the seismic data.
In borehole seismic acquisition, a tool 10 with receivers is located deep (e.g. 1 km) down a drilled well 12 below a rig 14 (see FIGS. 7a and b). The source 4 is located at the surface. Borehole seismic acquisition can be done either at sea or on land. The employed source arrays are usually smaller than in the previously mentioned types of seismic acquisition. A borehole seismic survey is usually called a Vertical Seismic Profile (VSP). An acquisition geometry of a 3D VSP in a vertical well at sea is shown in FIGS. 7a and b. It can be seen that the seismic data are acquired for all azimuths and the azimuthal directivity of the source array 4 will be present in the data. To a lesser degree it can also be present in a 2D VSP.
U.S. Pat. No. 5,142,498 seeks to construct arrays where the phase spectrum for all take-off angles of interest will match the phase spectrum of the vertically downgoing pulse. This is referred to as phase control. Phase control is achieved by symmetrically arranging identical source elements about the array's geometric centroid. The geometric centroid is the centre line in the source array about which the identical source elements are symmetrically arranged. This is the line where phase control is achieved. If all elements are equal, phase control is achieved in all azimuths for a range of take-off angles limited by geometry. However, phase control is only achieved within a limited range of take-off angles, and although the beam pattern is identical within the limited range of take-off angles where phase control is achieved, the beam pattern is not identical outside this limit.
The invention seeks to provide a seismic source array which is azimuth-invariant, in the sense that it emits a seismic wavefield whose change over a selected range azimuths is zero or negligible. Such a source array can then be used in multi-azimuth seismic acquisition.